gracefully handle errors for unsupported cabal version

plugins/hls-cabal-plugin/src/Ide/Plugin/Cabal.hs

@@ -65,6 +65,7 @@ import           Text.Regex.TDFA
 
 
 import qualified Data.Text                                     ()
+import           Distribution.Parsec.Error
 import qualified Ide.Plugin.Cabal.CabalAdd                     as CabalAdd
 
 data Log
@@ -247,7 +248,20 @@ cabalRules recorder plId = do
         let warningDiags = fmap (Diagnostics.warningDiagnostic file) pWarnings
         case pm of
           Left (_cabalVersion, pErrorNE) -> do
-            let errorDiags = NE.toList $ NE.map (Diagnostics.errorDiagnostic file) pErrorNE
+            let regex :: T.Text
+                -- We don't support the cabal version, this should not be an error, as the
+                -- user did not do anything wrong. Instead we cast it to a warning
+                regex = "Unsupported cabal-version [0-9]+.[0-9]*"
+                unsupportedCabalHelpText = "\nThe used cabal version is not fully supported by hls. This means that some functionallity might not work as expected.\nIf you face any issues try to downgrade to a supported cabal version."
+                errorDiags =
+                  NE.toList $
+                    NE.map
+                      ( \pe@(PError pos text) ->
+                          if text =~ regex
+                            then Diagnostics.warningDiagnostic file (Syntax.PWarning Syntax.PWTOther pos (text <> unsupportedCabalHelpText))
+                            else Diagnostics.errorDiagnostic file pe
+                      )
+                      pErrorNE
                 allDiags = errorDiags <> warningDiags
             pure (allDiags, Nothing)
           Right gpd -> do

plugins/hls-cabal-plugin/src/Ide/Plugin/Cabal/Diagnostics.hs

@@ -63,8 +63,9 @@ positionFromCabalPosition :: Syntax.Position -> Position
 positionFromCabalPosition (Syntax.Position line column) = Position (fromIntegral line') (fromIntegral col')
   where
     -- LSP is zero-based, Cabal is one-based
-    line' = line-1
-    col' = column-1
+    -- Cabal can return line 0 for errors in the first line
+    line' = if line <= 0 then 0 else line-1
+    col' = if column <= 0 then 0 else column-1
 
 -- | Create a 'FileDiagnostic'
 mkDiag

plugins/hls-cabal-plugin/test/Main.hs

@@ -107,6 +107,14 @@ pluginTests =
                     length diags @?= 1
                     unknownLicenseDiag ^. L.range @?= Range (Position 3 24) (Position 4 0)
                     unknownLicenseDiag ^. L.severity @?= Just DiagnosticSeverity_Error
+            ,   runCabalTestCaseSession "Publishes Diagnostics on unsupported cabal version as Warning" "" $ do
+                _ <- openDoc "unsupportedVersion.cabal" "cabal"
+                diags <- cabalCaptureKick
+                unknownVersionDiag <- liftIO $ inspectDiagnostic diags ["Unsupported cabal-version 99999.0"]
+                liftIO $ do
+                    length diags @?= 1
+                    unknownVersionDiag ^. L.range @?= Range (Position 0 0) (Position 1 0)
+                    unknownVersionDiag ^. L.severity @?= Just DiagnosticSeverity_Warning
             , runCabalTestCaseSession "Clears diagnostics" "" $ do
                 doc <- openDoc "invalid.cabal" "cabal"
                 diags <- cabalCaptureKick

plugins/hls-cabal-plugin/test/testdata/unsupportedVersion.cabal

@@ -0,0 +1,3 @@
+cabal-version:      99999.0
+name:               invalid
+version:            0.1.0.0